Generally, a motor vehicle automatic transmission includes a number of gear elements coupling its input and output shafts, and a related number of torque establishing devices, such as clutches and brakes, which are selectively engageable to activate certain gear elements for establishing a desired speed ratio between the input and output shafts.
The brake can be of the band type or disk type; engineering personnel in the automotive art refer to disc type brakes in transmissions as "clutches" or "reaction clutches". The input shaft is connected to the vehicle engine through a fluid coupling such as a torque converter and the output shaft is connected directly to the vehicle wheels. Shifting from one forward speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing or disengaging the clutch or brake (off-going) associated with the current speed ratio and applying or engaging the clutch or brake (on-coming) associated with the desired speed ratio.
Shifts performed in the above manner, using clutches and/or reaction clutches, are termed clutch-to-clutch shifts. Shifts of this type present particular control difficulty in situations where a downshift to a lower speed ratio is required to accelerate the vehicle or maintain speed up a grade. Such downshifts typically involve relatively high transmission input torque and are referred to herein as power-on downshifts.
The control difficulty concerns the relative timing of the off-going release and the on-coming apply. If the on-coming clutch is applied before the off-going clutch is released, the clutches oppose each other and the output torque is sharply reduced; if the off-going clutch is released before the on-coming clutch is applied, the transmission is effectively shifted to neutral and the engine speed flares. Both situations degrade the shift quality and are highly undesirable.
The shift control is additionally complicated by the fact that the on-coming clutch must be pressurized, or filled, with hydraulic fluid before it has the capacity to transmit torque. For the reasons outlined above, automatic transmissions commonly include free-wheeling or one-way torque transmitting elements in series with a reaction clutch, which elements are capable of transmitting torque in one direction only. As the on-coming clutch is applied and begins to transmit torque, the free-wheeling or one-way device associated with the off-going clutch overruns, effectively releasing the off-going clutch. However, the one-way devices do not come without expense, and it is highly desirable to limit their usage in transmissions.